Molecular sieves are a commercially important class of crystalline materials. They have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different species. Natural and synthetic crystalline molecular sieves are useful as catalysts and adsorbents. The adsorptive and catalytic properties of each molecular sieve are determined in part by the dimensions of its pores and cavities. Thus, the utility of a particular molecular sieve in a particular application depends at least partly on its crystal structure. Because of their unique sieving characteristics, as well as their catalytic properties, molecular sieves are especially useful in such applications as gas drying and separation and hydrocarbon conversion. The term “molecular sieve” refers to a material prepared according to the present invention having a fixed, open-network structure, usually crystalline, that may be used to separate hydrocarbons or other mixtures by selective occlusion of one or more of the constituents, or may be used as a catalyst in a catalytic conversion process. Zeolites are included in the term “molecular sieve”.
Prior art methods of preparing crystalline zeolites typically produce finely divided crystals which must be separated from an excess of liquid in which the zeolite is crystallized. The liquid, in turn, must be treated for reuse or else be discarded, with potentially deleterious environmental consequences. Preparing commercially useful catalytic materials which contain the powdered zeolite also normally requires additional binding and forming steps. Typically, the zeolite powder as crystallized must be mixed with a binder material and then formed into shaped particles or agglomerates, using methods such as extruding, agglomeration, and the like. These binding and forming steps greatly increase the complexity of catalyst manufacture involving, e.g., zeolitic materials. The additional steps may also have an adverse effect on the catalytic performance of the zeolite so bound and formed.
A number of processes have been offered for preparing crystalline zeolites within discrete particles. For example, Howell, et al., in U.S. Pat. No. 3,119,660 teaches a method for producing crystalline metal aluminosilicate zeolite by reacting preformed bodies of clay particles in an aqueous reactant mixture including alkali metal oxide. Similar processes for preparing zeolites from formed bodies, which may contain zeolitic seed crystals, in alkali solutions are also taught in U.S. Pat. No. 4,424,144 to Pryor, et al., U.S. Pat. No. 4,235,753 to Brown, et al., U.S. Pat. No. 3,777,006 to Rundell, et al., U.S. Pat. No. 3,119,659 to Taggart, et al, U.S. Pat. No. 3,773,690 to Heinze, et al., U.S. Pat. No. 4,977,120 to Sakurada, et al. and GB 2 160 517 A. U.S. Pat. No. 3,094,383 teaches a method of forming an A type zeolite by aging a homogeneous reaction mixture out of contact with an external aqueous liquid phase but under conditions to prevent the dehydration of the mixture. GB 1 567 856 discloses a method of preparing zeolite A by heating an extruded mixture of metakaolin powder and sodium hydroxide.
In U.S. Pat. No. 4,058,586, Chi, et al. discloses a method for crystallizing zeolites within formed particles containing added powdered zeolite, where the formed particles furnish all of the liquid needed for crystallization. Crystallizing the particles in an aqueous alkaline solution is not required using the process of Chi, et al.
Verduijn, in WO 92/12928, teaches a method of preparing binder-free zeolite aggregates by aging silica-bound extruded zeolites in an aqueous ionic solution containing hydroxy ions. According to the disclosure of Verduijn, the presence of zeolite crystals in the extrudate is critical for making strong crystalline zeolite extrudates. Verduijn, et al., in EPO A1/0,284,206, describe a method of preparing binderless zeolite L by forming silica and preferably 10–50 wt % performed zeolite L crystallites into particles, and then reacting the particles with an alkaline solution containing a source of alumina to form the zeolite L.
More recently, similar methods have been proposed for preparing high silica zeolitic materials. Conventional methods for preparing high silica materials, having a SiO2/Al2O3 molar ratio of greater than about 10, and more typically greater than about 20, typically involves crystallizing the zeolites from aqueous solution. For example, U.S. Pat. No. 3,702,886 to Argauer, et al., teaches a method of preparing ZSM-5 from a solution containing tetrapropyl ammonium hydroxide, sodium oxide, an oxide of aluminum or gallium, an oxide of silica or germanium, and water. The digestion of the gel particles is carried out until crystals form. The crystals are separated from the liquid and recovered.
EPO A2/0,156,595, discloses the preparation of crystalline zeolites having a silica to alumina mole ratio greater than 12 and a Constraint Index of 1 to 12 by forming a mixture of seed crystals, a source of silica, a source of alumina and water into shaped particles, which are then crystallized in an aqueous reaction mixture containing a source of alkali cations. It is also taught that alumina-containing clay may be used as an alumina source. U.S. Pat. No. 4,522,705 is directed to a catalytic cracking catalyst comprising an additive prepared by the in-situ crystallization of a clay aggregate disclosed in EPO A2/0,156,595.
Special methods for preparing the reaction mixture from which a zeolite may be crystallized have also been proposed. In U.S. Pat. No. 4,560,542 a dried hydrogel containing silica and alumina is contacted with a fluid medium containing an organic templating agent and maintained at specified crystallization conditions to form a crystalline aluminosilicate. In U.S. Pat. No. 5,240,892 a reaction mixture containing at least about 30 weight percent solids content of alumina and precipitated silica is taught for preparing zeolites. The method of preparing the reaction mixture allows agitation of the mixture during crystallization, in spite of the high solids content of the mixture.
Zeolite crystallization from reaction mixtures initially containing a gel-like phase in equilibrium with an excess of liquid phase is disclosed in R. Aiello, et al., “Zeolite Crystallization from Dense Systems”, Materials Engineering 1992, Vol. 3, n. 3, pp. 407–416.
Other approaches to synthesis of crystalline zeolites have included preparing the zeolites in an essentially aqueous-free environment. These non-aqueous methods have been described, for example, in ZEOLITES, 1992, Vol. 12, April/May, p. 343; ZEOLITES 1990, Vol. 10, November/December, p. 753; ZEOLITES 1989, Vol. 9, November, p. 468; Nature, Vol. 317(12), September 1985, p. 157; and J. Chem. Soc., Chem. Commun., 1988, p. 1486. J. Chem. Soc., Chem. Commun., 1993, p. 659 describes a kneading method for synthesizing ZSM-35 in a nonaqueous system, in which the amount of liquids used to prepare a crystallization mixture is not sufficient to wet all the solid particles so that the conglomerate reactant is actually a mixture of dry powder and small doughy lumps.
U.S. Pat. No. 6,004,527, issued Dec. 21, 1999 to Murrell et al. relates to the hydrothermal synthesis of large pore molecular sieves from nutrients, at least one of which contains an amorphous framework-structure, and which framework-structure is essentially retained in the synthetic molecular sieve. The synthesis involves impregnating a cation oxide framework comprising a first cation oxide with a liquid containing a second cation different from the first cation, said liquid being free of a pore forming agent. The impregnated cation oxide framework is dried and impregnated again with a liquid containing a pore forming agent. The amount of liquid containing the pore forming agent in the second impregnation does not exceed the incipient wetness point of the cation oxide framework. The impregnated cation oxide framework is then heated to produce a large pore molecular sieve.
U.S. Pat. No. 5,558,851, issued Sep. 24, 1996 to Miller, discloses a method for preparing a crystalline zeolite from a reaction mixture containing only enough water so the reaction mixture can be shaped if desired. The reaction mixture is heated at crystallization conditions and in the absence of an external liquid phase, so that excess liquid need not be removed from the crystallized material prior to drying the crystals.
U.S. Pat. No. 4,091,007, issued May 23, 1978 to Dwyer et al., discloses a method for preparing a crystalline aluminosilicate zeolite having uniform pores and greater than 40 percent crystallinity which comprises forming a critical reaction mixture containing a source of at least two cations, silica, alumina and water, wherein at least about 70 weight percent of the alumina is provided to the reaction mixture by an alumina-containing clay being added thereto. The reaction mixture is maintained at a temperature and pressure for a time necessary to crystallize the crystalline aluminosilicate. It is stated that it is desirable to preform the reaction mixture into discrete particles such as pellets or extrudates which retain their shape and acquire substantial strength in the crystallization process.
In Example 22, Dwyer et al. discloses the synthesis of ZSM-5 by mixing Georgia kaolin, Ludox colloidal silica and water. The mixture is dried in a Koline-Sanderson spray drier. More than 30% of the spray dried particles are larger than 200 mesh. The particles are calcined in air, and a portion of them mixed with a solution containing tetrapropylammonium bromide, NaOH pellets, Q-brand sodium silicate, NaCl and water. The resulting mixture is transferred to a static bomb and placed in a heated oil bath. Crystals are recovered and determined to be 50 weight percent crystalline ZSM-5.